The Structure of Camera Film

The Structure of Camera Film

Black-and-white films render the
naturally colored image that is projected upon them by the lens as a
monochromatic (that is, one-color) image: one composed of an infinitely
shaded series of grays whose lightness or darkness corresponds
inversely to the brightness of the light forming the image in those
areas. Such films consist of a sandwich of several layers bonded
together. First there is the film base, or medium of support-usually a
flexible plastic, but occasionally glass or a rigid plastic. The
emulsion deposited upon this consists of light-sensitive silver halides
suspended within a transparent, water-absorbent gelatin. Included in
the sandwich are layers of dyes; one of these dyes (the anti-halation
coating) absorbs the stray light scattered within the emulsion and film
base, and other, light-filtering dyes determine the basic color
sensitivity of the film.

Exposure to an image-that is, to light
focused on the film by the camera lens-causes physical changes in the
emulsion that establish a latent image, visible only after the film has
been chemically developed. During development the emulsion absorbs the
developing agents, and the portions of the light-sensitive compounds in
the emulsion that were affected by exposure to light are chemically
reduced to grains of metallic silver. These clump together roughly in
proportion to the amount of light that struck their area of the film;
in the aggregate, they make up the developed visible image. After
development, the image is made permanent by chemical fixing, which also
removes unexposed light-sensitive materials from the emulsion. At one
stage or another, all this chemistry also removes the anti-halation
layer and the color-sensitizing dyes. The result, after washing and
drying, is a negative: a reversed-tone, translucent image on the film.

So-called color films are actually
multiple-layered black-and-white films in which there are three or more
layers of emulsion, each sensitized and filtered so as to record only
certain wavelengths of light. In the course of processing, the silver
grains forming the various layered images are dissolved out and are
replaced by appropriately colored dyes. The result can be a color
negative, with all colors the chromatic reverse of those that will
appear on the color print; that is, blue color will appear yellow on
the negative, green will appear magenta, red will appear cyan, and so
on. Alternatively, reversal processing can provide a direct-positive
transparency-what we call a color slide-with colors rendered quite
naturally.

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